.box {
  display: flex;
}
.box .small {
  position: relative;
  width: 300px;
  height: 225px;
  cursor: move;
}
.box .small .small-img {
  display: block;
}
.box .small .mark {
  position: absolute;
  width: 100px;
  height: 100px;
  background-color: rgba(0, 0, 0, 0.3);
  top: 0;
  left: 0;
}
.box .big {
  margin-left: 20px;
  width: 300px;
  height: 300px;
  overflow: hidden;
  position: relative;
}
.box .big .big-img {
  position: absolute;
  top: 0;
  left: 0;
  display: block;
}
